Chemical mechanical polishing typically involves mounting a semiconductor wafer on a wafer holder. The wafer holder presses the wafer against an abrasive polishing surface which is moving with respect to the wafer. A chemical slurry is inserted between the wafer and the polishing surface. The chemical slurry includes abrasive particles which mechanically polish the wafer, as well as etchants which chemically remove material from the pad of the wafer. Typically, the wafer holder rotates the wafer as the wafer moves with respect to the polishing surface. This added motion increases the uniformity with which the wafer is polished. However, over time, the polishing surface may absorb the chemical slurry. This may change the properties of the abrasive surface over time.